1. Field of the Invention
The present invention relates to a technique to input coordinate information by pointing a position on an input surface.
2. Description of the Related Art
Conventionally, a coordinate input apparatus (for example, a touch panel apparatus), which inputs coordinate information when the user points a position on an input surface by his or her finger or a pointing device has prevailed. Based on the coordinate information input in this way, a connected computer can be controlled, or a character or figure can be written.
For example, it is a common practice to use a touch panel apparatus as a surface of, for example, a rear projector, front projector, or plasma display panel, and to display, for example, a screen of a personal computer (PC) on that surface. In such a system, the user can operate the PC by his or her finger or a pointing device via the touch panel apparatus. Also, such a system can realize a large-scale interactive display apparatus which allows the user to input handwritten traces. Such large-scale apparatus allows a usage method that is similar to a so-called whiteboard. For example, an application that displays a drawn trace runs on a PC connected to a display, and a trace input by the user by his or her finger or a pointing device can be displayed.
As a coordinate input method in the conventional coordinate input apparatus, various methods such as that using light, that using a resistive film, that using ultrasonic waves, and that using a capacitance are available. For example, as a coordinate input method using light, a light interruption method is known. In this method, a retro-reflective sheet is provided to the outer side of a coordinate input surface, and light projected from a light projecting unit is reflected by the retro-reflective sheet, and a light amount distribution is detected by a light receiving/detecting unit. According to this method, an angle of an area interrupted by an interrupting object such as a finger located within an input area is detected, thus detecting coordinates of the interrupted position, that is, an input position (for example, see U.S. Pat. No. 4,507,557, and Japanese Patent Laid-Open Nos. 2000-105671 and 2001-142642).
For example, according to U.S. Pat. No. 4,507,557, a peak value corresponding to a portion interrupted by an interrupting object in the light receiving unit is detected by a waveform processing operation such as a differential, thereby detecting an angle of the light-interrupted portion with respect to the light receiving unit. Then, based on this detection result, coordinates of the interrupting object are calculated. Also, Japanese Patent Laid-Open No. 2000-105671 describes a technique for detecting coordinates of one end and the other end of a light-interrupted portion by comparison with a specific level pattern, and detecting their center.
Since in the aforementioned use mode such as a whiteboard writing characters and drawing pictures are main purpose, an input to the coordinate input apparatus normally uses a pointing device that imitates a pen. The pointing device of this type (to be referred to as a pen hereinafter) has a switch at its pen tip. In case of a writing operation, when the pen tip is brought into contact with an input area of the coordinate input apparatus (to be referred to a main body hereinafter), that switch is enabled (ON). The ON/OFF operations of the switch notifies the main body of a pen down state (a state in which the pen is in contact with the input area) and device identification information (ID). Furthermore, the side surface of the pen (pen side) may also have switches to which various functions such as a function corresponding to a right button of a mouse and a page feed function are assigned.
When the pen tip emits light at a pressing timing of each switch, data is transmitted to the main body. When the coordinate input method of the main body is the aforementioned light interruption method, a light projection by the light projecting unit and a light emission of the pen may often occur simultaneously. In this case, it becomes difficult for the light receiving/detecting unit to detect a correct light-interrupted signal. Then, input coordinates cannot be calculated and, for example, a drawing line may be interrupted, resulting in poor operability.
In order to avoid such superposition of light emissions, a method of setting a light emission cycle of the light projection by the light projecting unit and that of the pen to be different from each other is known (for example, see Japanese Patent Laid-Open No. 2005-276019). According to this method, the light emission cycle of the pen is decided based on the light projection cycle and period of the light projection, a detection period of the light projection or pen light emission by the light receiving/detecting unit, and the light emission cycle of the pen light emission. Furthermore, the pen light emission cycle is controlled not to match an integer multiple of the light projection cycle. The detection result by the light receiving/detecting unit is established after a plurality of times of light projections. Then, the pen light emission and light projection can be avoided from occurring at the same time, thus preventing missing of any pen signal detection.
In the aforementioned use mode such as a whiteboard in, for example, a meeting room, it is desired to allow a plurality of operators to simultaneously make inputs. Hence, a coordinate input apparatus that copes with a plurality of simultaneous inputs has also been designed.
However, when inputs using a plurality of pens are to be made on the coordinate input apparatus, a plurality of pen down events may occur simultaneously. In this case, the following problems are posed. In general, the pen light emission cycle starts simultaneously with a pressing event of each of switches (pen tip switch and pen side switches), and ends simultaneously with a pen up event (the pen is separated from the input area). Therefore, even when the plurality of pens respectively have different light emission cycles, if simultaneous pen down events of the plurality of pens occur, their light emissions start at the same time. That is, the initial light emissions occur at the same time. When the simultaneous light emissions have occurred in this way, since light signals each indicating up/down information and device identification information of a pen are superposed with each other, correct bit sequences cannot be restored. For this reason, since an ID required to identify each pen cannot be assigned to detected coordinates in the initial light emissions, a delayed coordinate output, that is, a low writing response occurs.
Even in the method of setting different light emission cycles of the light projecting unit and pen in Japanese Patent Laid-Open No. 2005-276019, that light emission cycle starts simultaneously with a pressing event of the pen tip switch or pen side switch, and ends simultaneously with a pen up event. That is, this method cannot avoid superposition of the initial light emissions at the time of pen down events. Therefore, this method is premised on a delay of coordinate detection, that is, a low response at the beginning of writing (writing response).
The aforementioned problems of superposition of light emissions due to the simultaneous pen down events and that of the main body light projection and pen light emission are caused because light is emitted simultaneously with pen down events, since new light emission cycles start at the time of the pen down events after pen up events.
Hence, in order to avoid superposition of light emissions of the pens when the plurality of pens are used, and that of the pen light emission and main body light projection, a method of controlling to set different light emission timings by synchronizing the main body of the coordinate input apparatus with the plurality of pens may be used. However, when they are connected by cables, for example, in order to synchronize the main body with the pens, free movements of the pens are impaired, resulting in poor input operability. Also, a method of always exchanging sync signals wirelessly may be used. However, in this case, the service life of a battery especially on the pen side becomes so short as to pose a practical issue.